Apoptosis is an important process in the development of cells and is important in maintaining the proper number of cells in the body. Candidates for apoptosis include cells that may be a danger to an organism, such as cells with damaged DNA or cells growing at improper rates. However, apoptosis is also applied to normal cells that have simply become obsolete as organisms grow and develop.
Bcl-2 protein is known to inhibit apoptotic cell death. Bcl-2 protein serves as a check on apoptosis allowing healthy and useful cells to survive. Anti-apoptotic molecules, such as Bcl-2 are often overexpressed in cancer cells and their inhibition is an attractive target for selective killing of tumor cells via induction of apoptosis. Bcl-2 overexpression and/or activation has been correlated with resistance to chemotherapy, to radiotherapy and to development of hormone-resistant tumors. Inhibition of apoptosis by Bcl-2 contributes to cancer by inhibiting cell death. Thus, inhibiting Bcl-2 activity in cancer cells can reduce chemotherapeutic resistance and increase the killing of cancer cells.
The Bcl-2 gene was discovered as the translocated locus in a B-cell leukemia. Bcl-2 contains a single transmembrane domain and is localized within a cell to the outer mitochondrial, nuclear, and endoplasmic reticulum membranes. Bcl-2 was first isolated as a breakpoint rearrangement in human follicular lymphomas. In humans, most follicular B-cell lymphomas contain a chromosomal translocation that moves the gene for Bcl-2 from its normal location to a position within the genes for immunoglobulins. In this new location, higher quantities of Bcl-2 are produced. Since Bcl-2 is a potent pro-survival protein, it shields the cancer cells from apoptotic instruction.
The effector molecules in the apoptotic pathway are a family of enzymes known as the caspases. The Bcl-2 protein suppresses apoptosis by preventing the activation of the caspases that carry out the process. Caspase enzymes are cystein proteases that selectively cleave proteins at sites just C-terminal to aspartate residues. These proteases have specific intracellular targets such as proteins of the nuclear lamina and cytoskeleton. The cleavage of these substrates leads to the demise of a cell.
The inositol 1,4,5-triphosphate (IP3) messenger molecule is water soluble, and can diffuse within the cytosol carrying an activated G protein signal from the cell surface to the endoplasmic reticulum (ER) surface. IP3 binds to an IP3R and induces opening of the channel allowing Ca2+ ions to exit from the ER into the cytosol. The released calcium then triggers a mass exodus of cytochrome c from all mitochondria in the cell, thus activating the caspase and nuclease enzymes that finalize the apoptotic process.
It has previously been shown that Bcl-2 interacts with the inositol 1,4,5-triphosphate receptor (IP3R) and inhibits IP3-mediated Ca2+ release from the ER, thereby inhibiting anti-CD3 induced apoptosis in immature T cells (JCB 166:193-203,2004; JCB 172: 127-137, 2006). IP3R have a broad tissue distribution and are mostly found in the cell integrated into the endoplasmic reticulum. The IP3R is a large six transmembrane ligand gated ion channel which mainly transmits calcium ions and thereby facilitates triggers apoptosis.